**5. PASK contribution to hepatic adaptation to fasting/feeding**

The liver maintains metabolic homeostasis, and it is especially essential in the proper control of glucose during fasting and feeding periods. In particular, the liver is one of the main insulin-responsive organs, so it records a greater glucose uptake when glycaemia rises, storing it as glycogen (see Section 2.1). By contrast, when blood glucose falls, and other organs require energy, the liver produces glucose by glycogenolysis and gluconeogenesis. Therefore, the correct hepatic response to insulin and hepatic metabolism are critical for maintaining glycaemia within physiological ranges, and therefore for the proper control of diabetes.

Studies with PASK−/− mice have reported the critical role PASK plays in hepatic adaptation to fasting/feeding periods, especially under an HFD [93, 112]. It is interesting that PASK expression is regulated in the liver by fasting/feeding, with fasting downregulating it [112]. Moreover, Perez-Garcia et al. [113] have found that PASK deficiency alters the complex hepatic response to fasting/feeding. The expression of the transcription factors and key enzymes that regulate gluconeogenesis and mitochondrial fatty acid transport under fasting conditions is altered in PASK−/− mice, with lower forkhead box protein O1 (*Foxo 1*) and carnitine palmitoyltransferase 1A (*Cpt1a*) and higher peroxisome proliferator-activated receptor alpha (*Ppara*). Similarly, PASK deficiency modifies the activity of the protein kinase B (AKT) overactivated under fasting and the stability of phosphoenolpyruvate carboxykinase (PEPCK) [113], while no detectable changes have been observed in the maintenance of blood glucose homeostasis during prolonged fasting periods [105].

A good example of PASK deficiency effects under feeding involves the changes recorded in GCK, which is a critical enzyme in the hepatic function. GCK is an enzyme involved in hepatic glucose sensing (see Section 2). This enzyme is activated by the increase in blood glucose which occurs in feeding periods. It therefore adjusts hepatic glucose phosphorylation to blood glucose levels, acting as a glucose sensor. The importance of GCK in maintaining glucose homeostasis is evidenced by the severe impacts caused by mutations in the GCK gene. The loss of GCK function in the human body causes maturity-onset diabetes of the young type 2 (MODY2) [114]. By contrast, activating mutations generate persistent hyperinsulinemia [115]. Many liver functions are controlled by GCK, which acts together with insulin in the maintenance of blood glucose homeostasis [116], and the activation of glycolytic and lipogenic gene expression. GCK is also involved in glycogen synthesis and storage in the liver [117]. The enzymatic activity of GCK is controlled by transcriptional and posttranscriptional mechanisms. While the transcriptional regulation of the GCK gene is basically insulin-dependent [118], the posttranscriptional mechanisms of regulation involve interaction with other proteins, highlighting the glucokinase regulatory protein (GCKR). GCKR modulates GCK activity when glucose levels decline by binding and sequestering it in the nucleus, and thus avoiding its function in the cytoplasm [116, 119].

Studies relating the role of PASK to the GCK function [113] have revealed that GCK activity is reduced in PASK**−/−** mice for two reasons: on the one hand, the lower protein expression, and on the other, its mainly nuclear location. It cannot be ruled out that the decrease in GCK may be partly due to the blocking of lipogenesis that characterizes PASK**−/−** mice. In addition, the conversion of excess carbohydrates into lipids might also be limited by the low levels of acetyl CoA carboxylase (ACC) and fatty acid synthase (FAS), although the gene coding to ACC and liver pyruvate kinase (LPK) is overexpressed under non-fasted conditions in PASK**−/−** mice. Additionally, glycogen metabolism could also be modified because glycogen synthase is a PASK substrate [90].

The hepatic PASK role in the control of hepatic adaptation to fasting/feeding becomes more important under an HFD because it dysregulates hepatic metabolic responses to fasting and feeding, leading to a non-alcoholic fatty liver, obesity, insulin resistance, diabetes, and associated cardiovascular problems. Some studies have evidenced that PASK**−/−** mice fed with a HFD resist the development of obesity and hepatic steatosis, with improved insulin sensitivity [93, 96, 112]. A consequence of an HFD for the liver is that it alters the downregulation of the *Pask* expression produced by fasting, as normally happens in a standard-fat diet [112]. Interestingly, PASK**−/−** mice with an HFD record improved parameters for the following: body weight, glucose tolerance, insulin resistance (see Section 6.1.), and serum lipid parameters [112]. Some of the PASK effects are due to changes in the proteolytic maturation of SREBP1c, others by regulating transcription factors and enzymes that play a key role in the hepatic response to fasting/feeding. Thus, PASK deficiency compensates for gene expression altered by an HFD. For example, it decreases the expression of genes overexpressed in HFD-fed mice (transcription factors involved in the regulation of gluconeogenic enzymes, the transport of fatty acid into mitochondria, β-oxidation, and *de novo* lipogenesis). PASK also modifies the expression of the short noncoding RNAs involved in lipid metabolism and glucose homeostasis. Such is the case of miR-33a and miR-143, whose expression in HFD-fed mice is controlled by PASK. Thus, PASK deficiency improves the hepatic adaptation to feeding/fasting, especially under pathogenic situations such as an HFD, through a highly regulated molecular mechanism that controls the expression and function of the transcription factors, enzymes and miRNAs involved in glucose and insulin signaling.

PASK deficiency also improves oxidative metabolism and mitochondrial biogenesis [120], increasing the ROS-detoxifying enzymes and the expression of *FoxO3a* and PTEN-induced kinase 1 (PINK1) involved in cell survival and mitophagy, respectively. All of these are interesting effects of PASK deficiency for states that increase oxidative stress, such as aging, diabetes, and obesity.

In sum, there are several results that highlight PASK's role in the control of the key genes and proteins that lead to hepatic metabolic adaptation to fasting or feeding situations.

Accordingly, PASK has been proposed as one of the possible targets for the treatment of the metabolic syndrome.

#### **6. PASK and insulin resistance**

The growing interest in the PASK function began with the finding that its deficiency prevents many of the deleterious effects of HFDs [93, 112], with a highlight being the insulin resistance that accompanies these diets, and which has been widely associated with the development of T2D [121, 122].

**355**

*the expression of PEPCK and G6P genes.*

**Figure 3.**

*Role of Nutrient and Energy Sensors in the Development of Type 2 Diabetes*

The same level of PASK is expressed in pancreatic α and β cells, and it is involved in insulin and glucagon secretion [88]. PASK promotes insulin expression [108], while PASK deficiency decreases the expression of preproinsulin at high glucose concentrations [123]. As for glucagon, PASK regulates its secretion by

PASK is also important in the development of pancreatic α and β cells, reflecting its key role in diabetes [89]. Thus, even though PASK deletion does not affect glucose-stimulated insulin secretion or insulin levels in response to fasting and feedback, it does decrease pancreatic β-cell mass in specific KO animals. By contrast, the deletion of PASK in pancreatic α cells improves glucagon secretion both *in vivo* and *in vitro*. PASK therefore plays a clear role in glucagon secretion and in the

There are two determining factors that promote the onset of T2D, dysfunctions in insulin secretion and peripheral resistance to the action of insulin. We cannot speak of a single class of T2D because there is considerable heterogeneity. In general, obesity prompts the metabolic syndrome [124], which in addition to obesity includes other pathologies such as hypertension, hypertriglyceridemia, elevated fasting glucose levels, and dyslipidemia. T2D is associated with insulin resistance (**Figure 3**), and over long periods it can lead to hyperinsulinemia. Finally,

Insulin signaling activates the PI3K/AKT pathway that controls most metabolic effects (**Figure 3**) [125]. Insulin stimulates glucose uptake in muscle and adipose tissue, promoting glucose transporter type 4 (GLUT4) expression and translocation to the cell membrane [126, 127]. In turn, insulin decreases lipolysis in adipose tissue,

*Insulin signaling and insulin resistance. Insulin signaling recruit insulin receptor substrates (IRS) activates the phosphoinositide 3-kinase (PI3K)/AKT pathway that controls most metabolic effects. In muscle and adipose tissue this pathway activates AMP-activated protein kinase (AMPK) promoting glucose transporter type 4 (GLUT4) expression and translocation to the cell membrane. In the liver this pathway activates glucokinase (GCK) and glycogen synthase kinase (GSK) inducing glycogen synthesis and suppresses gluconeogenesis inhibiting the expression of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). Arrow stimulates, T bar red inhibits. Insulin resistance impairs the activation of PI3K/Akt pathway. In the skeletal muscle and adipose tissue impairs glucose uptake decreasing GLUT4 expression and translocation. In the liver insulin resistance suppresses glycogen synthesis and promotes gluconeogenesis through* 

*DOI: http://dx.doi.org/10.5772/intechopen.95454*

development of β-cell precursors.

the pancreas fails and hypoinsulinemia sets in.

glucose [88].

#### *Role of Nutrient and Energy Sensors in the Development of Type 2 Diabetes DOI: http://dx.doi.org/10.5772/intechopen.95454*

The same level of PASK is expressed in pancreatic α and β cells, and it is involved in insulin and glucagon secretion [88]. PASK promotes insulin expression [108], while PASK deficiency decreases the expression of preproinsulin at high glucose concentrations [123]. As for glucagon, PASK regulates its secretion by glucose [88].

PASK is also important in the development of pancreatic α and β cells, reflecting its key role in diabetes [89]. Thus, even though PASK deletion does not affect glucose-stimulated insulin secretion or insulin levels in response to fasting and feedback, it does decrease pancreatic β-cell mass in specific KO animals. By contrast, the deletion of PASK in pancreatic α cells improves glucagon secretion both *in vivo* and *in vitro*. PASK therefore plays a clear role in glucagon secretion and in the development of β-cell precursors.

There are two determining factors that promote the onset of T2D, dysfunctions in insulin secretion and peripheral resistance to the action of insulin. We cannot speak of a single class of T2D because there is considerable heterogeneity. In general, obesity prompts the metabolic syndrome [124], which in addition to obesity includes other pathologies such as hypertension, hypertriglyceridemia, elevated fasting glucose levels, and dyslipidemia. T2D is associated with insulin resistance (**Figure 3**), and over long periods it can lead to hyperinsulinemia. Finally, the pancreas fails and hypoinsulinemia sets in.

Insulin signaling activates the PI3K/AKT pathway that controls most metabolic effects (**Figure 3**) [125]. Insulin stimulates glucose uptake in muscle and adipose tissue, promoting glucose transporter type 4 (GLUT4) expression and translocation to the cell membrane [126, 127]. In turn, insulin decreases lipolysis in adipose tissue,

#### **Figure 3.**

*Type 2 Diabetes - From Pathophysiology to Cyber Systems*

synthase is a PASK substrate [90].

and insulin signaling.

ing situations.

treatment of the metabolic syndrome.

**6. PASK and insulin resistance**

Studies relating the role of PASK to the GCK function [113] have revealed that GCK activity is reduced in PASK**−/−** mice for two reasons: on the one hand, the lower protein expression, and on the other, its mainly nuclear location. It cannot be ruled out that the decrease in GCK may be partly due to the blocking of lipogenesis that characterizes PASK**−/−** mice. In addition, the conversion of excess carbohydrates into lipids might also be limited by the low levels of acetyl CoA carboxylase (ACC) and fatty acid synthase (FAS), although the gene coding to ACC and liver pyruvate kinase (LPK) is overexpressed under non-fasted conditions in PASK**−/−** mice. Additionally, glycogen metabolism could also be modified because glycogen

The hepatic PASK role in the control of hepatic adaptation to fasting/feeding becomes more important under an HFD because it dysregulates hepatic metabolic responses to fasting and feeding, leading to a non-alcoholic fatty liver, obesity, insulin resistance, diabetes, and associated cardiovascular problems. Some studies have evidenced that PASK**−/−** mice fed with a HFD resist the development of obesity and hepatic steatosis, with improved insulin sensitivity [93, 96, 112]. A consequence of an HFD for the liver is that it alters the downregulation of the *Pask* expression produced by fasting, as normally happens in a standard-fat diet [112]. Interestingly, PASK**−/−** mice with an HFD record improved parameters for the following: body weight, glucose tolerance, insulin resistance (see Section 6.1.), and serum lipid parameters [112]. Some of the PASK effects are due to changes in the proteolytic maturation of SREBP1c, others by regulating transcription factors and enzymes that play a key role in the hepatic response to fasting/feeding. Thus, PASK deficiency compensates for gene expression altered by an HFD. For example, it decreases the expression of genes overexpressed in HFD-fed mice (transcription factors involved in the regulation of gluconeogenic enzymes, the transport of fatty acid into mitochondria, β-oxidation, and *de novo* lipogenesis). PASK also modifies the expression of the short noncoding RNAs involved in lipid metabolism and glucose homeostasis. Such is the case of miR-33a and miR-143, whose expression in HFD-fed mice is controlled by PASK. Thus, PASK deficiency improves the hepatic adaptation to feeding/fasting, especially under pathogenic situations such as an HFD, through a highly regulated molecular mechanism that controls the expression and function of the transcription factors, enzymes and miRNAs involved in glucose

PASK deficiency also improves oxidative metabolism and mitochondrial biogenesis [120], increasing the ROS-detoxifying enzymes and the expression of *FoxO3a* and PTEN-induced kinase 1 (PINK1) involved in cell survival and mitophagy, respectively. All of these are interesting effects of PASK deficiency for states that

In sum, there are several results that highlight PASK's role in the control of the key genes and proteins that lead to hepatic metabolic adaptation to fasting or feed-

Accordingly, PASK has been proposed as one of the possible targets for the

The growing interest in the PASK function began with the finding that its deficiency prevents many of the deleterious effects of HFDs [93, 112], with a highlight being the insulin resistance that accompanies these diets, and which has been

increase oxidative stress, such as aging, diabetes, and obesity.

widely associated with the development of T2D [121, 122].

**354**

*Insulin signaling and insulin resistance. Insulin signaling recruit insulin receptor substrates (IRS) activates the phosphoinositide 3-kinase (PI3K)/AKT pathway that controls most metabolic effects. In muscle and adipose tissue this pathway activates AMP-activated protein kinase (AMPK) promoting glucose transporter type 4 (GLUT4) expression and translocation to the cell membrane. In the liver this pathway activates glucokinase (GCK) and glycogen synthase kinase (GSK) inducing glycogen synthesis and suppresses gluconeogenesis inhibiting the expression of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). Arrow stimulates, T bar red inhibits. Insulin resistance impairs the activation of PI3K/Akt pathway. In the skeletal muscle and adipose tissue impairs glucose uptake decreasing GLUT4 expression and translocation. In the liver insulin resistance suppresses glycogen synthesis and promotes gluconeogenesis through the expression of PEPCK and G6P genes.*

reducing the level of circulating free FA [128]. Insulin in the liver induces glycogen synthesis and suppresses gluconeogenesis [75].

The loss of sensitivity to insulin, generally called insulin resistance, affects this hormone's main peripheral target organs [125]. Insulin resistance could increase the concentration of free FA in the blood [129], decrease glucose uptake in skeletal muscle, and stimulate gluconeogenesis, increasing glucose release in the liver, thereby contributing to the metabolic syndrome [130, 131]. Abdominal obesity releases an excess of free FA, and the associated inflammation may interfere with insulin signaling [132].

#### **6.1 Obesity, insulin resistance and PASK**

Obesity in humans is defined as a body mass index (BMI = weight/height<sup>2</sup> ) of more than 30 kg/m<sup>2</sup> . There are numerous studies linking fat intake and the onset of T2D, but it has not been possible to establish a consensus on the relationship between fat intake and obesity. A positive correlation has nonetheless been established between fat consumption and the BMI by Nagao et al. [121]. Moreover, the effect of consuming animal fats that contain high amounts of saturated fatty acids (SFAs) has also been compared with the consumption of fat with monounsaturated (MUFA) or polyunsaturated fatty acids (PUFA). PUFA intake is reported to reduce the risk of T2D. The variability detected also depends on the genetic factors that contribute 40% to the development of T2D [121].

Genetic variability is also considered in studies on the effect of an HFD in mice [121, 133]. In sum, a diet containing high amounts of animal or vegetable fat rich in saturated or unsaturated FA ω6/ω9 increases body weight and resistance to insulin appears, and to compensate there is an increase in insulin levels. These effects are not observed when using oils rich in unsaturated FA ω3 [134].

The liver's key function of maintaining metabolic homeostasis in both fasting and a postprandial state makes it one of the main organs affected by an HFD. It has been posited that a long-term HFD induces lipid (triglycerides) accumulation in hepatocytes as a result of insulin resistance. Nonalcoholic fatty liver disease (NAFLD) is the first step toward the onset of a chronic condition. This first step is followed by oxidative stress and impairment of the mitochondrial function, which triggers associated inflammation, hepatic damage and fibrosis [135, 136].

PASK−/− mice are protected against hepatic steatosis and the insulin resistance induced by an HFD [93]. We have reported that a long-term HFD severely alters the liver response needed to maintain metabolic homeostasis during fasting and feeding periods [112]. Firstly, the hepatic *Pask* expression is stimulated by feeding [96]. By contrast, HFD-fed mice have similar levels of hepatic *Pask* gene expression under fasting and feeding conditions [112]. Our results suggest that this effect might be responsible for some of the metabolic changes associated with this diet.

An HFD has a drastic effect on the expression of transcription factors that regulate the adaptation to fasting/feeding conditions. For instance, the transcription factors (FOXO1, peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α) and PPARα) stimulated under fasting to promote the expression of the gluconeogenic enzymes (PEPCK and glucose 6-phosphatase (G6Pase)) and the genes that support fatty acid transport into mitochondria and β-oxidation are significantly overstimulated in HFD-fed mice. However, PASK deficiency blocks or diminishes the expression of all these genes under an HFD.

Similarly, the adaptation to a postprandial state is regulated by glucose and insulin through the expression of transcriptional factors (carbohydrate-responsive element-binding protein (CHREBP), liver X receptor alpha (LXRα) and *SREBP1*) [137, 138]. They promote the expression of lipogenic genes (*Acc1*, *Fas* and

**357**

activity [144].

affects liver function [147].

*Role of Nutrient and Energy Sensors in the Development of Type 2 Diabetes*

stearoyl-CoA desaturase-1 (*Scd1*)), stimulating the conversion of excess carbohydrates into FA and triglycerides. An HFD also overexpresses both *Lxr*α and peroxisome proliferator-activated receptor gamma (*Ppar*γ) but this effect is diminished in PASK−/− mice [112]. The overexpression of *Lxr*α [139] and *Ppar*γ [140, 141] has previously been associated with liver steatosis in human and mouse models of

The effects of an HFD cause multiple changes in transcription factors and the key enzymes of the main hepatic metabolic pathways that allow metabolic adaptation. Genes promoting de novo lipogenesis that are expressed only under feeding conditions with a standard diet can also be overexpressed under fasting conditions with an HFD, which induces the overexpression of transcription factors and metabolic enzyme genes controlling de novo lipogenesis (*Chrebp*, *Lxr*α, *Srebp1c*, *Acc1* and

PASK deficiency eliminates many harmful effects that HFDs cause in the liver (**Figure 3**), also decreasing the lipid depots that over time can develop hepatic steatosis. Thus PASK−/− mice under an HFD have lower blood glucose levels, improve their sensitivity to the action of insulin, preventing the appearance of insulin resistance, which in turn is correlated with smaller increases in body weight and improved lipid profile [112]. PASK pharmacologic inhibition likewise confirms its key role for restoring insulin sensitivity, and for reducing hepatic fat content and

The risk of T2D increases in aging due to the many imbalances that characterize this stage of life, which are often associated with overweight, impaired glucose

The aging process is characterized metabolically by the following: the development of insulin resistance (**Figure 3**), changes in body composition and mitochondrial dysfunction. In addition, hyperinsulinemia and glucose intolerance develop during aging [143, 144]. There are numerous metabolic changes in peripheral tissues that affect the uncontrolled gluconeogenesis of the liver, accompanied by an increase in lipogenesis in adipose tissue, and by defects in glycogen synthesis and glucose uptake in skeletal muscle [144]. Glucose metabolic dysfunction is closely correlated with oxidative stress, as occurs in diabetic or obese patients [145, 146]. Aging is normally accompanied by an increase in visceral fat, which is one of the main contributors to insulin resistance and the development of T2D. Likewise, it leads to an increase in proinflammatory cytokines, which interfere with insulin

Another consequence of aging is the progressive loss of mitochondrial function in various tissues such as the liver or skeletal muscle. Thus, certain studies affirm that there is an association in aging between insulin resistance and glucose intolerance, together with a reduction in oxidative activity and mitochondrial ATP synthesis [144]. With aging, the liver undergoes molecular changes such as an increased inflammatory response, dysregulation of the genetic expression of antioxidant enzymes, and mitochondrial dysfunction, significantly altering redox homeostasis. It is also accompanied by the liver's reduced capacity for regeneration, which greatly

The nutrient sensing mechanisms needed to detect and respond to variations in their levels are dysregulated by aging [148]. So both nutrient sensor pathways, AMPK and mTORC1, are involved in a lifespan [149]. PASK senses intracellular oxygen, redox state and various metabolites [100]. Additionally, PASK regulates both AMPK and mTORC1 pathways [104, 105]. The aging process at cellular level is

*DOI: http://dx.doi.org/10.5772/intechopen.95454*

obesity and diabetes.

*Scd1*) under a fasted state [112].

the fibrosis caused by an HFD [98].

**6.2 Aging, insulin resistance and PASK**

metabolism, hypertension and dyslipidemia [142].

#### *Role of Nutrient and Energy Sensors in the Development of Type 2 Diabetes DOI: http://dx.doi.org/10.5772/intechopen.95454*

*Type 2 Diabetes - From Pathophysiology to Cyber Systems*

synthesis and suppresses gluconeogenesis [75].

**6.1 Obesity, insulin resistance and PASK**

contribute 40% to the development of T2D [121].

not observed when using oils rich in unsaturated FA ω3 [134].

insulin signaling [132].

more than 30 kg/m<sup>2</sup>

reducing the level of circulating free FA [128]. Insulin in the liver induces glycogen

Obesity in humans is defined as a body mass index (BMI = weight/height<sup>2</sup>

of T2D, but it has not been possible to establish a consensus on the relationship between fat intake and obesity. A positive correlation has nonetheless been established between fat consumption and the BMI by Nagao et al. [121]. Moreover, the effect of consuming animal fats that contain high amounts of saturated fatty acids (SFAs) has also been compared with the consumption of fat with monounsaturated (MUFA) or polyunsaturated fatty acids (PUFA). PUFA intake is reported to reduce the risk of T2D. The variability detected also depends on the genetic factors that

Genetic variability is also considered in studies on the effect of an HFD in mice [121, 133]. In sum, a diet containing high amounts of animal or vegetable fat rich in saturated or unsaturated FA ω6/ω9 increases body weight and resistance to insulin appears, and to compensate there is an increase in insulin levels. These effects are

The liver's key function of maintaining metabolic homeostasis in both fasting and a postprandial state makes it one of the main organs affected by an HFD. It has been posited that a long-term HFD induces lipid (triglycerides) accumulation in hepatocytes as a result of insulin resistance. Nonalcoholic fatty liver disease (NAFLD) is the first step toward the onset of a chronic condition. This first step is followed by oxidative stress and impairment of the mitochondrial function, which

PASK−/− mice are protected against hepatic steatosis and the insulin resistance induced by an HFD [93]. We have reported that a long-term HFD severely alters the liver response needed to maintain metabolic homeostasis during fasting and feeding periods [112]. Firstly, the hepatic *Pask* expression is stimulated by feeding [96]. By contrast, HFD-fed mice have similar levels of hepatic *Pask* gene expression under fasting and feeding conditions [112]. Our results suggest that this effect might be

triggers associated inflammation, hepatic damage and fibrosis [135, 136].

responsible for some of the metabolic changes associated with this diet.

diminishes the expression of all these genes under an HFD.

An HFD has a drastic effect on the expression of transcription factors that regulate the adaptation to fasting/feeding conditions. For instance, the transcription factors (FOXO1, peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α) and PPARα) stimulated under fasting to promote the expression of the gluconeogenic enzymes (PEPCK and glucose 6-phosphatase (G6Pase)) and the genes that support fatty acid transport into mitochondria and β-oxidation are significantly overstimulated in HFD-fed mice. However, PASK deficiency blocks or

Similarly, the adaptation to a postprandial state is regulated by glucose and insulin through the expression of transcriptional factors (carbohydrate-responsive element-binding protein (CHREBP), liver X receptor alpha (LXRα) and *SREBP1*)

[137, 138]. They promote the expression of lipogenic genes (*Acc1*, *Fas* and

. There are numerous studies linking fat intake and the onset

) of

The loss of sensitivity to insulin, generally called insulin resistance, affects this hormone's main peripheral target organs [125]. Insulin resistance could increase the concentration of free FA in the blood [129], decrease glucose uptake in skeletal muscle, and stimulate gluconeogenesis, increasing glucose release in the liver, thereby contributing to the metabolic syndrome [130, 131]. Abdominal obesity releases an excess of free FA, and the associated inflammation may interfere with

**356**

stearoyl-CoA desaturase-1 (*Scd1*)), stimulating the conversion of excess carbohydrates into FA and triglycerides. An HFD also overexpresses both *Lxr*α and peroxisome proliferator-activated receptor gamma (*Ppar*γ) but this effect is diminished in PASK−/− mice [112]. The overexpression of *Lxr*α [139] and *Ppar*γ [140, 141] has previously been associated with liver steatosis in human and mouse models of obesity and diabetes.

The effects of an HFD cause multiple changes in transcription factors and the key enzymes of the main hepatic metabolic pathways that allow metabolic adaptation. Genes promoting de novo lipogenesis that are expressed only under feeding conditions with a standard diet can also be overexpressed under fasting conditions with an HFD, which induces the overexpression of transcription factors and metabolic enzyme genes controlling de novo lipogenesis (*Chrebp*, *Lxr*α, *Srebp1c*, *Acc1* and *Scd1*) under a fasted state [112].

PASK deficiency eliminates many harmful effects that HFDs cause in the liver (**Figure 3**), also decreasing the lipid depots that over time can develop hepatic steatosis. Thus PASK−/− mice under an HFD have lower blood glucose levels, improve their sensitivity to the action of insulin, preventing the appearance of insulin resistance, which in turn is correlated with smaller increases in body weight and improved lipid profile [112]. PASK pharmacologic inhibition likewise confirms its key role for restoring insulin sensitivity, and for reducing hepatic fat content and the fibrosis caused by an HFD [98].

#### **6.2 Aging, insulin resistance and PASK**

The risk of T2D increases in aging due to the many imbalances that characterize this stage of life, which are often associated with overweight, impaired glucose metabolism, hypertension and dyslipidemia [142].

The aging process is characterized metabolically by the following: the development of insulin resistance (**Figure 3**), changes in body composition and mitochondrial dysfunction. In addition, hyperinsulinemia and glucose intolerance develop during aging [143, 144]. There are numerous metabolic changes in peripheral tissues that affect the uncontrolled gluconeogenesis of the liver, accompanied by an increase in lipogenesis in adipose tissue, and by defects in glycogen synthesis and glucose uptake in skeletal muscle [144]. Glucose metabolic dysfunction is closely correlated with oxidative stress, as occurs in diabetic or obese patients [145, 146].

Aging is normally accompanied by an increase in visceral fat, which is one of the main contributors to insulin resistance and the development of T2D. Likewise, it leads to an increase in proinflammatory cytokines, which interfere with insulin activity [144].

Another consequence of aging is the progressive loss of mitochondrial function in various tissues such as the liver or skeletal muscle. Thus, certain studies affirm that there is an association in aging between insulin resistance and glucose intolerance, together with a reduction in oxidative activity and mitochondrial ATP synthesis [144]. With aging, the liver undergoes molecular changes such as an increased inflammatory response, dysregulation of the genetic expression of antioxidant enzymes, and mitochondrial dysfunction, significantly altering redox homeostasis. It is also accompanied by the liver's reduced capacity for regeneration, which greatly affects liver function [147].

The nutrient sensing mechanisms needed to detect and respond to variations in their levels are dysregulated by aging [148]. So both nutrient sensor pathways, AMPK and mTORC1, are involved in a lifespan [149]. PASK senses intracellular oxygen, redox state and various metabolites [100]. Additionally, PASK regulates both AMPK and mTORC1 pathways [104, 105]. The aging process at cellular level is regulated by insulin/IGF signaling and both AMPK and mTORC1 pathways, which are in turn regulated by nutrient levels, whose signals converge on several targets: FOXO, nuclear factor: erythroid-derived 2-like 2 (NRF2), tumor protein p53, and sirtuins (SIRT) in order to control metabolic homeostasis, oxidative stress, and quality cellular housekeeping [150].

PASK−/− mice may avoid several of the deleterious defects induced by the aging process [151]. Aged PASK−/− mice maintain both low blood glucose values and insulin concentrations similar to young WT mice. They do not develop glucose intolerance or insulin resistance, as confirmed by a normal HOMA-IR index. These effects correlate with a high expression of the longevity gene *FoxO3a* and the transcription factor NRF2, as the main regulator of the redox balance [151]. Signaling through the system NRF2/KEAP1: kelch-like ECH-associated protein 1 regulates the transcription of enzymes that protect cells against oxidative stress [152]. An elevated expression of glutamate-cysteine ligase modifier subunit (GCLm) and heme oxygenase-1 (HO1) have been found in aged PASK−/− mice under fasted conditions. The efficiency of this redox system decreases in step with aging in WT mice, significantly diminishing the antioxidant response [153]. Likewise, PASK deficiency prevents the drastically age-related decrease in the expression of several antioxidant enzymes under basal conditions, such as catalase (CAT) and glutathione peroxidase (GPx) [151].

In relation to the maintenance of the mitochondrial function and energy homeostasis, we have confirmed that the expression of the several transcription factors and nuclear receptors needed to maintain mitochondrial biogenesis (*Ppargc1a, Sirt1* and *Nrf2)* are affected in fasted aged WT mice [151] in accordance with the previous literature that relates aging to a decrease in cellular energy input [154], an increase in oxidative stress [155], and the mitochondrial dysfunction of cellular redox homeostasis [156, 157]. However, the expression of *Nrf2, Ppargc1a, Ppar*γ and *Sirt1* increases under fasting in aged PASK−/− mice. This means they maintain lower levels of ROS/RNS, while aged WT mice record a lower expression of antioxidant enzymes and increased levels of ROS/RNS [151].

We might therefore contend that some of the dysfunctions produced during aging in PASK−/− mice could be related to hormetic responses. Slight toxic effects can generate beneficial actions that compensate for the initial damage and even improve cellular health [158]. Aging decreases *Pask* expression in the liver of WT mice perhaps as a compensatory mechanism. PASK−/− mice maintain the same blood glucose values as young WT mice, and do not develop insulin resistance.
